1. Field of the Invention
The present invention relates generally to a vacuum monitor for detecting poor vacuum pressure within a vacuum interrupter, and more specifically to a vacuum monitor which can produce an alarm or indicate a caution when vacuum pressure within a vacuum interrupter is abnormally increasing.
2. Description of the Prior Art
Generally, a vacuum interrupter for use with an electric power circuit has a normal circuit interruption performance when pressure of vacuum within its evacuated envelope is kept below 10.sup.-4 Torr (Torricelli). However, the pressure of vacuum sometimes increases and the circuit interruption performance deteriorates, because of, for instance, outgassing from materials used for the interrupter or slow leakage of air (air is leaked through cracks caused by undue mechanical stresses or through welded or insufficiently brazed junction portions). In case vacuum pressure increases abnormally within the evacuated envelope, the small contact spacing will no longer by able to sustain a high voltage applied to the contacts; arcs and flashovers will occur; white hot arc will burn the contact surfaces and may melt the vacuum envelope and other parts of the vacuum interrupter.
Therefore, in an electric power circuit, it is very important to check or monitor vacuum pressure within the vacuum interrupter while a vacuum interrupter is in operation; namely, even while the contacts are kept closed or opened. In recent years, accordingly, various kinds of vacuum pressure measuring devices for vacuum interrupters have been put into practical use. These vacuum pressure measuring devices, however, have the following disadvantages in practical use: (1) There exists a vacuum pressure measuring device such that a pair of electric discharge electrodes are additionally provided within the evacuated envelope and a high voltage is independently applied to the electric discharge electrodes from a separate high-voltage power supply, in order to check the change in electric discharge phenomenon caused in accordance with Paschen's law (because vacuum pressure exerts an influence upon the phenomenon of electric discharge). In such a device, the structure of the vacuum interrupter is complicated and the manufacturing cost is relatively high, because an additional high-voltage power supply is necessary. (2) There exists another vacuum pressure measuring method such that a vacuum interrupter is once disconnected from a power circuit and the movable and fixed contacts are readjusted to such an appropriate spacing that electric discharge is readily generated in accordance with Paschen's law, before applying a high voltage thereto from a separate high voltage power supply, in order to check the state of electric discharge. In such a method, the power circuit or the power supply must be disconnected from the vacuum interrupter; that is, it is impossible to check the vacuum pressure while applying a high voltage to the interrupter (i.e. under hot-line condition) and therefore it takes much time and troublesome labor. (3) Further, recently miniature substation installations have been developed within which a vacuum interrupter is housed. In such an installation, it is necessary to drain insulation medium such as oil or gas out of the installation housing before checking the vacuum pressure within the vacuum interrupter, provided that the method described above (2) is employed. In such an installation, accordingly, more time and more troublesome labor are required, and additionally, there exists another possibility of the occurrence of accidents caused by erroneous reassembly or readjustment due to human mistake.
To overcome the above-mentioned shortcomings there has been proposed a vacuum monitor for a vacuum interrupter which can easily check poor vacuum pressure within a vacuum interrupter, without need of any additional electric discharge electrodes or high voltage power supply, provided that a high supply voltage is being applied to a power circuit via a vacuum interrupter, at relatively low manufacturing cost and through simple checking procedure. This vacuum monitor for a vacuum interrupter comprises an antenna disposed near a conductive material of the vacuum interrupter for receiving impulsive electromagnetic wave signals generated by electric discharge caused in accordance with a prebreakdown voltage depending upon Paschen's law when vacuum pressure increases within the vacuum interrupter and a detector circuit connected to the antenna for electrically processing the impulsive electromagnetic wave signals received by the antenna in order to indicate poor vacuum pressure within a vacuum interrupter. The vacuum monitor as described above is disclosed in detail in our former Patent Application Document (U.S. patent application Ser. No. 437,678 filed on Oct. 29, 1982, EPC Patent Application No. 82 305 761.7 filed on Oct. 29, 1982, or Korean Patent Application No. 82-4846 filed on Oct. 28, 1982).
However, since this vacuum monitor can check poor vacuum pressure within the vacuum interrupter in dependence upon impulsive electromagnetic wave signals emitted to the air from the vacuum interrupter or a conductive material connected to the interrupter, in the case where the vacuum interrupter is perfectly covered by an insulating solid material and the insulating solid material is additionally covered by a metal layer connected to the ground, no impulsive electromagnetic wave signals are emitted to the outside. In the same way, in the case where the vacuum interrupter is housed within a metal housing filled with an insulating medium such as an oil or gas and the metal housing is perfectly grounded and further where other conductive materials connected to the vacuum interrupter are all shielded perfectly, no impulsive electromagnetic wave signals are emitted to the outside. In other words, there exist shortcomings such that it is impossible to check poor vacuum pressure within a vacuum interrupter which is housed within a casing perfectly shielded and grounded.
Furthermore, in the case where an antenna is used for detecting the impulsive electromagnetic wave signals, even if the vacuum interrupter is not housed within a casing; that is, the vacuum interrupter is bare, since the voltage level of the impulsive electromagnetic wave signals detected through the antenna is relatively low, a higher-gain amplifier circuit is inevitably needed at the succeeding stage, thus resulting in a higher-cost in the vacuum monitor for a vacuum interrupter.